WO2018015722A1

WO2018015722A1 - Non-destructive testing apparatus

Info

Publication number: WO2018015722A1
Application number: PCT/GB2017/052090
Authority: WO
Inventors: Julian Roger Wright; Timothy David MOTTRAM
Original assignee: Theta Technologies Ltd
Current assignee: Theta Technologies Ltd
Priority date: 2016-07-18
Filing date: 2017-07-14
Publication date: 2018-01-25
Anticipated expiration: 2019-01-18
Also published as: GB2555770A; GB2555770B; GB201612434D0

Abstract

A non-linear acoustic non-destructive testing apparatus for detecting sub-surface discontinuities comprising having at least one transducer; a driving circuit communicably coupled to said at least one transducer; and an output circuit for determining at least one parameter of the acoustic contact between said at least one transducer and said surface of a workpiece in use. The apparatus is configured to have a workpiece-test mode, wherein said transducer is disconnected from said output circuit, and an impedance measurement mode, wherein said transducer is connected to said output circuit.

Description

NON-DESTRUCTIVE TESTING APPARATUS

This invention relates to a non-linear acoustic non-destructive testing apparatus. More specifically, though not necessarily exclusively, the invention relates to apparatus which tests for defects in samples of materials which have been subject to stress forces, or improperly manufactured.

In the field of non-destructive testing techniques, hereinafter referred to as NDT, it is known to use acoustic methods in order to analyse the sub-surface structure of a sample which may have been subject to stress or may exhibit manufacturing flaws. For example, NDT may be used to examine structures which have been stir-welded, or structures formed from composite materials. Acoustic NDT generally transmits ultrasonic sound waves via an ultrasound transducer through the depth of a sample at regular intervals along its planar surface. Data about the resultant waves is collected by a receiving transducer (which may be integral with the transmitter). The data may then be stored in digital format, and processed to provide resulting data, such as a graph or a map, to be displayed on a screen. The transducers will typically scan across the entire upper surface of a sample in order to collect comprehensive data about the inner structure.

There are, however, many problems with the practical application of such methods of NDT. Since the technique relies on the successful transmission of ultrasonic energy into, and out of, the sample there is significant scope for error and/or noise to be introduced into the data due to impedance mismatches or variations in coupling conditions. For example, reflection at the boundary between the transmitter and the upper surface of the sample to be tested attenuates the energy of the transmitted ultrasound waves, meaning that they may not be able to penetrate as far. Additionally, such reflected waves may be picked up by the ultrasonic receiver, thus obfuscating the data. In order to alleviate or reduce such problems, a couplant may be provided in order to match more closely the impedance between the air and the sample, so that a larger proportion of the wave energy is transmitted into the sample. Such a couplant may be gel or water based and therefore is moveable about the sample's surface. However, this may pose a problem wherein the thickness or stiffness of the couplant is repeatedly changing throughout the testing process as the transducer moves along the sample's surface. Thus, the coupling may not only be uneven, but may also fluctuate during testing. Acoustic wave energy will attenuate somewhat through such a layer of couplant, and the amount of attenuation depends on the thickness of the couplant. If this is variable, then the data collected has a variable error which, in prior art arrangements, cannot be predicted.

Embodiments of the present invention aim to address at least some of the above problems. In accordance with a first aspect of the invention there is provided a non-linear acoustic non-destructive testing apparatus for detecting sub-surface discontinuities comprising: - at least one transducer (which may be a transmitter and/or receiver); a driving circuit communicably coupled to said transducer; and an output circuit for determining at least one parameter of the acoustic contact between said at least one transducer and said surface of a workpiece in use, said apparatus being configured to have a workpiece-test mode, wherein said transducer is disconnected from said output circuit, and a impedance measurement mode, wherein said at least one transducer is connected to said output circuit.

In one exemplary embodiment of the present invention, the apparatus may further comprise a motorised positional system movable along at least one dimensional plane. The positioning system may be communicably coupled to the driving circuit, and programmable to move throughout a pre-defined testing area. The transducer, or transducers (which may be a transmitter and/or receiver), may be mounted to the positioning system.

The apparatus may be configured to automatically switch between said workpiece-test mode and said impedance measurement mode. Optionally, the positional system may be movable in all three dimensions.

According to one exemplary embodiment the apparatus may further comprise an external motor controller board (MCB) communicable to said positional system. The apparatus may further comprise an external signal generation and data capture module.

Optionally, the apparatus may further comprise an internal frequency generator. The apparatus may further comprise an amplification unit. The amplification unit may receive data from an external signal generation and data capture module and output data to a transducer and/or receiver in workpiece-test mode. The amplification unit may receive data from said internal frequency generator in said impedance measurement mode.

In one exemplary embodiment the apparatus may further comprise a programmable microcontroller unit communicably coupled to an external control unit. The external control unit might be a desktop computer, or other user friendly computer.

The amplification unit may be driven by either the motor controller board (MCB) or the microcontroller unit. The microcontroller unit and the external control unit might communicate via a universal serial bus (USB) communications chip. According to an exemplary embodiment of the present invention, the output circuit may comprise a complex resistor network, a voltage divider and a radio frequency integrated circuit. The data may be output to a memory storage unit. The data from said output circuit may be sent via the microcontroller to the computer memory storage unit.

Optionally, the complex resistor network may comprise a plurality of resistors with varying resistance, where the output circuit being may be able to switch between the resistors in accordance with varying transducers. In accordance with a second aspect of the invention there is provided a method of nonlinear acoustic non-destructive testing comprising; providing at least one transducer; coupling the at least one transducer to a workpiece via a coupling medium; transmitting acoustic energy into the workpiece through the coupling medium; monitoring the resultant signals within the test sample; analysing the received data to identify non-linearities in the time domain indicative of discontinuities within the sample, characterised in that the method further comprises: monitoring the quality of the coupling at the sensor coupling-test sample interface during the NDT procedure.

According to a further exemplary embodiment of the invention, the step of monitoring the resultant signals may comprise measuring or determining the impedance conditions at the interface between two media.

Optionally, the electrical impedance may be monitored by detection of current and voltage to provide an indication of acoustic impedance changes. The monitoring may be automatic, and could be set to interval or continuous.

In one exemplary embodiment of the present invention, the method may further comprise providing a coupling quality parameter to the user based upon the monitoring. Additionally, the method may further comprise flagging to a user if the coupling quality is below a predetermined threshold. In accordance with a third aspect of the present invention there is provided a non-linear acoustic non-destructive testing apparatus for detecting surface or sub-surface discontinuities comprising: at least one transducer (may be a transmitter and/or a receiver); a driving circuit communicably coupled to at least one transducer in use for receiving resultant acoustic signals from the workpiece; and a processor arranged to process data from the output circuit to identify non-linearities in the frequency domain within the resulting signals which may be indicative of defects in the workpiece, characterised in that the apparatus further comprises: an acoustic impedance detector configured to detect and/or monitor the acoustic impedance at the interface between the transducer and the workpiece.

The method or apparatus of embodiments may be particularly applicable to linear acoustic NDT. Thus, embodiments may provide a linear acoustic non-destructive testing apparatus and a method of linear acoustic non-destructive testing.

These and other aspects of the present invention will be apparent from the following specific description, in which embodiments of the invention are described, by way of examples only, and with reference to the following drawings, in which:

Figure 1A is a schematic plan-view diagram of an acoustic NDT apparatus according to one exemplary embodiment of the invention;

Figure IB is a schematic cross-sectional side-view of the acoustic NDT apparatus of Figure 1A;

Figure 2 is a schematic diagram showing the basic configuration of the main components of the apparatus of Figure 1A; Figure 3 is a schematic circuit diagram showing the configuration of the impedance measurement board as set-up in the NDT apparatus of Figure 1.

Referring to Figures 1A and IB of the drawings, a positional system 120 is provided with a transducer/receiver 101 mounted at thereto. A Commercial-off-the-Shelf (COTS) servomotor (not shown) comprising a motor and a sensor for positional feedback provides the positional system 120 with multi-directional movement.

The positional system comprises two tubular bars 121 substantially perpendicular to one another. Each tubular bar 121 is affixed at each end to a motorized track member 122. The track members 122 are mounted to the upper rim of the test area walls 123. The test area walls 123 define a finite test area 124 between them which is generally rectangular in shape. Each tubular bar 121 is mounted to opposing walls 123. Each opposing pair of track members 122 move in unison along the rim of corresponding walls 123. A transducer is fixed to the point where the two tubular bars 121 cross, and is moveable along the vertical axis in use. The transducer may comprise a single ultrasonic transmitter/receiver or separate transducers (which could be on a single body). This means that the transducers may be positioned at any point within the range of the positional system 120. The transducers can be programmed to move in a scanning motion through the test area 124, during which ultrasonic waves are transmitted into a work piece and resulting signals measured at regular time intervals. The intervals may be as small or large as is required for the particular testing.

During the NDT process a sample of material to be tested for defects, or workpiece is placed in the test area 124. The workpiece 125 must have at least one substantially planar surface, and is oriented such that the useable work surface faces upwards. A couplant 126 is applied to the surface of the workpiece. The couplant 126 may be a gel or water based couplant, as is known in the art, and helps to ensure a good transmission of the ultrasonic waves from the transducer into the workpiece through the upper planar surface of the workpiece. The transducer 101 (see Figure 3) is positioned at a point on the surface of the workpiece 125, for example at a corner, such that the operational surface of the transducer is in contact with the couplant 126, and the positional system 120 begins to move the transducer across the surface of the workpiece 125. Ultrasonic energy is transmitted through the depth of the workpiece 125 and the resulting ultrasonic waves formed in the workpiece may be used to identify and or detect discontinuities. In particular, it has been found that by identifying any discontinuities (for example, non-linearities) in the resulting signal, voids or defects can be identified. The resulting signals are received, and data for example relating to the frequency, phase and wave speed is stored in electronic format for processing. As the transducer moves, the couplant 126 is displaced, and areas of thicker and thinner couplant arise, causing variable attenuation of the ultrasonic waves as discussed in the introductory portion of this document. If the couplant is too thick the signals indicative of a defect may have a much reduced energy and therefore are less easily distinguishable from the noise of other reflected waves. This may bring an element of uncertainty into the data collected. If the couplant is too thin, then the transducer/receiver may not be adequately coupled to the surface of the workpiece. This may allow the waves to be reflected at the upper surface such that test results become unreliable. The invention as described in this exemplary embodiment can move along two horizontal axes. However, depending upon the intended application other embodiments of the invention may comprise a different number of axes through which the transducer can be programmed to move, (for example, a single axis or using a six-axis system), and the invention is not intended to be limited in this regard. In order to measure the effect and/or alleviate the effect that coupling variation might be having on the NDT test results, embodiments of the present invention provide an apparatus and a method of testing the contact condition of the transducer 101. In other words, embodiments enable the user to be aware of how well the transducer is coupled to the surface of the workpiece. One property of the ultrasonic waves which changes with the coupling of the transducer to the surface of the workpiece is the acoustic or mechanical impedance. The applicants have identified that measuring the electrical impedance of a transducer allows changes in acoustic or mechanical impedance to be inferred.

In order for the electrical impedance to be calculated in 'real-time' and to accurately reflect the conditions of the NDT process, it is required that the system is able to measure the electrical impedance at intervals during the NDT process. To do this the apparatus as a whole may be arranged to be able to switch between the impedance testing system and the NDT system, and so there are two defined modes for the apparatus, a workpiece test mode and an impedance measurement mode.

Referring now to Figure 2, the electrical components of the NDT apparatus 100 comprise a motor controller board, hereinafter referred to as an MCB 114, which controls the directional movement of the track members 122 provided by the servomotor. When the apparatus is set in workpiece test mode, a COTS signal generator 110 transmits electronic signals to the transducer/receiver 101. The transducer 101 converts the signals into acoustic waves which are transmitted out beyond the operational surface of the transducer 101. In order to obtain clearer results, it is preferred that the acoustic waves transmitted are high energy, therefore the signals generated by the external signal generator need to be amplified. Accordingly, a COTS amplification unit 102 is provided. This will ensure that the resultant waves in the workpiece still have a strong enough signal to be distinguishable from any noise signals. The amplification unit 102 needs to have a gate signal to enable its output and this gate signal is provided by the MCB 112 during NDT. In this particular embodiment, the COTS signal generator is part of an external module which also comprises data capture hardware. The information acquired from the transducer when in workpiece testing mode is collected here.

When the apparatus is set up for measuring the electrical impedance, a first switch 103 changes the path of amplified signals from the amplification unit 102 to the impedance measurement board 105. The measurement board 105 comprises a resistor network 116, a potential divider, 107 and a radio frequency integrated circuit 109. In this operational setup, the transducer/receiver 101 is now only connected to the impedance measurement board 105 in series. A second switch 104 is provided to change the signal source of the amplification unit 102 from the COTS signal generator 110 to an internal frequency generator 108. The internal frequency generator 108 may be controlled by an internal microcontroller unit 111. A third switch 112 changes the source of the gate signal to the amplification unit 102 from the MCB 114 to the microcontroller unit 111. This means that both the frequency signal and the amplification signal are controlled by the microcontroller unit 111 which allows both the signals to be synchronised. The microcontroller unit 111 is programmable from an interface, typically a personal computer such as a personal computer (PC) 115. The computer and microcontroller may communicate using USART (Universal Synchronous/Asynchronous Receive/Transmit) protocol via a universal serial bus (USB) communication unit 113. As the microcontroller unit 111 is controlling all electrical components of the apparatus 100 it has a plurality of analogue to digital converters (ADCs) in order to reduce the design complexity of the apparatus. The USB 113 comprises a communications chip which connects to a standard USB 4-line interface. The chip used in this particular exemplary embodiment of the invention is connectable to USART transmit and receive lines on the microcontroller unit 111, and so a microcontroller unit 111 which has this functionality is used.

The amplification unit 102 used in this particular embodiment of the present invention is a TOMCO® radio frequency amplifier, though any suitable amplifier may be used, and indeed there is a large range of suitable amplifiers. The frequency generator is required to generate signals from as low as 20 kHz to 15 MHz for the current embodiment of the present invention, (though it will be understood by those skilled in the art that other frequency ranges can be used, and the invention is not intended to be limited in this regard.) In this embodiment, a Direct Digital Synthesis (DDS) component has been used, as it can be programmed to transmit specific frequencies and waveforms. Because of the large range of suitable amplifiers which operate over different frequency ranges, a pre- amplification unit 106 is provided.

Referring additionally to Figure 3 of the drawings, the resistor network 116 comprises a network of resistors, and the transducer/resistor 101. The input to the resistor network 116 from the amplification unit 102 is split along two branches; a first branch comprising the two resistors of known value, Ri and R₂, and a second branch comprising the transducer 101 and a third resistor, R₃ resistor of a value equal to the combined value of the resistors in the first branch of the impedance measurement board 105, i.e. Ri + R₂ = R₃. Each branch has its own output channel, Channel A, and Channel B. Channel A is the output channel for the first branch and Channel B being the output for the second branch. Channel A and Channel B provide data about the voltage difference and phase shift of the incoming signals from the amplification unit 102 respectively. The outputs from Channel A and Channel B may have voltages which can be too large to safely pass through to the other components of the NDT apparatus 100, and so the potential divider 107 is used to provide a voltage drop. The Radio Frequency Integrated Circuit (RFIC) 109 collects the outputs from the impedance measurement board 105 and measures the amplitude and phase differences of the two inputs. This RFIC 109 has a maximum allowable voltage and thus the need for the potential divider 107. The RFIC 109 will need to have its own power source which may be provided externally (not shown).

The RFIC is used in this system to measure the gain and phase of the two voltages input to the RFIC. The information generated by the RFIC 109 is sent back to the microcontroller unit 111, whereupon it may then be transferred to a memory storage unit in a computer 115 for processing at a later date. Various mathematical functions are applied to the data collected and a measurement of the electrical impedance of the transducer at that particular point during the test process as a whole is taken.

In use, the NDT apparatus will switch between the workpiece-testing mode and the impedance measurement mode automatically at intervals which may be programmable by a PC 115, and controlled by the microcontroller unit 111. These intervals will take place during the acoustic NDT process. As such, a 'map' of the electrical impedance of the transducer (i.e. a measure of the contact quality) may be built up over the entire testing process. This 'map' provides a point of reference when analysing the results of the workpiece test and can be used to asses the quality of the results. Alternatively or additionally, the impedance measurement may be used to indicate where significant problems may be arising during the workpiece testing or to flag less reliable results. The data is not necessarily used as a feedback loop, to make real-time adjustments during the NDT process, though it will be clear to a person skilled in the art that it may be used as such. The measure of contact quality can indicate a number of different contributing factors, with the thickness of the couplant, and therefore the attenuation of the wave energy, being one. Another is the condition of the operational surface of the transducer/receiver 101, or the condition of the surface of the workpiece.

Experiments were carried out to verify the method for measuring the electrical impedance of the transducer, and therefore quantifying the contact quality between the transducer and the upper surface of the workpiece, in accordance with an embodiment of the invention. The experimental procedures are described below, by way of example only.

Firstly, a simple, known electrical impedance of a test circuit was measured and compared with the theoretical data in order to ascertain the reliability of any results calculated. To do this a test circuit 200 was devised.

Referring to Figure 4 of the drawings, a resistor Ri 202 and capacitor Ci of known values are joined in series to make a complex network 204. In the experiment conducted, Ri = 180Ω (Ohms) was used as this was estimated to give a midpoint result between the expected values of the impedance of the transducer. Ci = lOOnF (Nano-Farads) was chosen because of the large phase angle this would yield in the results, therefore making it easy to observe. A theoretical value for the impedance and the phase angle were calculated using:

Z = R +—,

jwC

where w = 2nf

In the above equation Z is the complex impedance, R is the resistance (in this case 180Ω), C is the capacitance (100nF), is the frequency given in Hertz of the voltage applied to the circuit, w is the angular frequency, and j is the complex number V— 1. One can calculate the complex impedance by taking the modulus of Z, and the phase angle is calculated by taking the argument of Z:

Impedance can be measured by adding a further fixed resistor R₂ 203 of known resistance to the test circuit 200. The circuit 200 has a voltage input of VA, the voltage dropped across the known resistor 203 is Vi, and the voltage dropped across the complex network 204 is V_z. The capacitor can be represented theoretically as an equivalent complex resistor jX_x

201, where j is V— 1 and X_x is the complex reactance. Referring to Figure 5 of the drawings, representing the voltages in a vector diagram with complex voltages perpendicular to the real voltages, one can calculate the phase shift as the angle φ between the vector representative of VA and the vector representative of Vz. It can be seen that the angle Θ can be calculated using the law of cosines. Vectors V₂oi and V₂o₂ are representative of the voltages across the resistor 201 and the complex resistor 202 representing the capacitor. Using known mathematical methods, it can be shown that:

and:

Using these formulae, it is possible to calculate a value for the impedance of the simple complex network 204, and the results gained from this simple test calculation came within 1% of expected values.

In the setup described above, whilst values for complex impedance can be calculated, neither side of the resistor R₂ is grounded. This is an issue in that the voltage dropped across here: Vi, cannot be measured by a device which has the same ground reference as the signal source. One way of overcoming this is to change the circuit. Another is to measure the phase shift between V_z and VA instead of measuring the voltage across R_2. Trigonometry then provides a calculated value for Vi. This is the purpose for calculating the phase shift between the two input voltages to the RFIC 109 in the exemplary embodiment of the invention described with reference to Figure 2 of the drawings. Referring to Figure 6 of the drawings, a second test circuit 220 is provided to allow the above principle to be applied wherein the voltages are at high frequency. A power splitter 223 splits the signal from the load voltage source and two signal paths are created, one with a resistive path, and a second with a complex resistive path as shown in Figure 5 of the drawings. The voltage V_z is measured at point 221 and the voltage i.e. half the input voltage, is measured at point 222. Points 221 and 222 correspond with the output channels in the resistor network 116 as described in the exemplary embodiment with reference to Figure 3. The phase shift in this experimental environment is easily measured using a standard workshop oscilloscope 224, as long as the oscilloscope has two input channels. In this set up, Vi can be calculated using the law of cosines.

Having verified that the impedance is known to be measurable within an acceptable degree of accuracy, the applicants were able to introduce a transducer 101 to the test circuit, 220, in place of the complex resistor network 204 in order to determine if changes in electrical impedance indicate changes in acoustic impedance.

Next, the applicants generated an input signal using a generic signal generator, and a PC based oscilloscope was used to determine values for the root-mean-square of the current and also a value for φ, i.e. the phase shift between the two output voltages. Referring to Figures 6 and 7 of the drawings, a transducer 101 replaces the complex network 204 in the test circuit 220. Specific values for each of the transducer components is not required for the purposes of the invention, and so overall resistive and reactive components only are calculated. The impedance of the circuit 220 with the transducer 101 will change as the input frequency changes, and there will be a resonance and anti-resonance frequency, at which points the magnitude of the impedance will be a minimum and a maximum respectively. At frequencies below resonance and above anti-resonance, the net reactance of the circuit will be positive and therefore the circuit is inductive. At frequencies between the resonance and anti-resonance the net reactance of the circuit will be negative and therefore the circuit can be said to be capacitive.

The graphs shown in Figures 7-22 of the drawings represent the results from four separate tests. Two transducers were tested, with each transducer being tested independent of any workpiece (free-air coupled) and with a couplant applied between the operational surface of the transducer and the surface of the workpiece. In these tests a piece of carbon-fibre panel was used as the workpiece. Graphs of frequency against impedance, phase angle, resistance and reactance were plotted and the results are discussed below.

Figures 7- 10 of the drawings give the results for a 54kHz transducer being tested in free air. Frequencies of 43kHz to 100kHz in steps of 1kHz were tested using the setup described above. It can be seen that the minimum impedance, and therefore the resonance frequency, occurred at just below 54kHz. Referring to Figures 8 and 10 in particular, the switch from inductive reactance to capacitive reactance can be seen showing that at the anti-resonance frequency, the transducer becomes insensitive due to high impedance.

Referring to Figures 11-14 of the drawings, the 54kHz is tested with a couplant. The most notable difference in results when compared with the test done in reference to Figures 7-10 of the drawings, is that the anti-resonance frequency has reduced, while the resonance frequency remains approximately the same. This can be seen best by comparing Figures 10 and 14. This means that there is a larger range of frequencies for which the reactance is inductive, i.e. a larger range in which the transducer cannot work as efficiently because the impedance is too high. Referring to Figures 15-18 of the drawings, a 250kHz transducer is tested in free air. The frequency range used was 230kHz to 300kHz in steps of 2kHz. Similar features can be seen in this set of graphs as with the graphs in Figures 7-10. A resonance frequency can be seen to be in the vicinity of 250kHz.

Referring to Figures 19-22, the 250kHz transducer was tested with couplant. When compared with the results from the 54kHz with-couplant test, the most notable difference is that, here, the resonance frequency has increased, with the anti-resonance frequency remaining the same, therefore narrowing the range of frequencies for which the reactance is inductive.

From these results it is clear to see that changing the acoustic impedance has a substantial impact on those parameters which are recorded in order to calculate the electrical impedance of the system. Thus, the skilled person will readily appreciate the system as described in the test set-up can be used to as acoustic or mechanical impedance detection device in relation to non-linear acoustic NDT apparatus.

It will be appreciated by a person skilled in the art, from the foregoing description, that modifications and variations can be made to the described embodiments without departing from the scope of the invention as defined in the appended claims.

Claims

1. A non-linear acoustic non-destructive testing apparatus for detecting sub-surface discontinuities comprising: at least one transducer; a driving circuit communicably coupled to said at least one transducer; and an output circuit for determining at least one parameter of the acoustic contact between said at least one transducer and said surface of a workpiece in use, said apparatus being configured to have a workpiece-test mode, wherein said transducer is disconnected from said output circuit, and a impedance measurement mode, wherein said transducer is connected to said output circuit.

2. A non-linear acoustic non-destructive testing apparatus according to claim 1, further comprising a motorised positional system movable along at least one dimensional plane, said transducer being mounted thereto, communicably coupled to said driving circuit, and programmable to move throughout a pre-defined testing area.

3. A non-linear acoustic non-destructive testing apparatus according to claim 1 or claim 2, wherein said apparatus is configured to automatically switch between said workpiece-test mode and said impedance measurement mode.

4. A non-linear acoustic non-destructive testing apparatus according to any of claims 1 to 3, wherein said positional system is movable in all three dimensions.

5. A non-linear acoustic non-destructive testing apparatus according to any of claims 2 to 4, further comprising an external motor controller board (MCB) communicable to said positional system.

6. A non-linear acoustic non-destructive testing apparatus according to any of claims 1 to 5, further comprising an external signal generation and data capture module.

7. A non-linear acoustic non-destructive testing apparatus according to claim 6, further comprising an internal frequency generator.

8. A non-linear acoustic non-destructive testing apparatus according to claim 7, further comprising an amplification unit, said amplification unit receiving data from said external signal generation and data capture module and outputting data to said transducer and/or receiver in said workpiece-test mode, and said amplification unit receiving data from said internal frequency generator in said impedance

measurement mode.

9. A non-linear acoustic non-destructive testing apparatus according to any of the preceding claims, further comprising a programmable microcontroller unit communicably coupled to an external control unit.

10. A non-linear acoustic non-destructive testing apparatus according to claim 9,

wherein said amplification unit can be driven by either said motor controller board (MCB) or said microcontroller unit.

11. A non-linear acoustic non-destructive testing apparatus according to claim 10, wherein said microcontroller unit and said external control unit communicate via a universal serial bus communications chip.

12. A non-linear acoustic non-destructive testing apparatus according to any of the preceding claims, wherein said output circuit comprises a complex resistor network, a voltage divider and a radio frequency integrated circuit, and outputs data to a memory storage unit.

13. A non-linear acoustic non-destructive testing apparatus according to claim 13, wherein said data from said output circuit is sent via said microcontroller to said computer memory storage unit.

14. A non-linear acoustic non-destructive testing apparatus according to claim 13, wherein said complex resistor network comprises a plurality of resistors with varying resistance, said output circuit being able to switch between said resistors in accordance with varying transducers.

15. A method of non-linear acoustic non-destructive testing comprising; providing at least one transducer; coupling the at least one transducer to a workpiece via a coupling medium; transmitting acoustic energy into the workpiece through the coupling medium; monitoring the resultant signals within the test sample; analysing the received data to identify non-linearities in the time domain indicative of discontinuities within the sample, characterised in that the method further comprises: monitoring the quality of the coupling at the sensor coupling-test sample interface during the NDT procedure.

16. A method of non-linear acoustic non-destructive testing according to claim 17, wherein the step of monitoring the resultant signals comprises measuring or determining the electrical impedance at the interface.

17. A method of non-linear acoustic non-destructive testing according to claim 18, wherein the electrical impedance is monitored by detection of current and voltage to provide an indication of acoustic impedance changes.

18. A method of non-linear acoustic non-destructive testing according to claim 19, wherein he monitoring is automatic, and could be set to interval or continuous.

19. A method of non-linear acoustic non-destructive testing according to any of the preceding claims, further comprising providing a coupling quality parameter to the user based upon the monitoring.

20. A method of non-linear acoustic non-destructive testing according to any of the preceding claims, wherein the method further includes flagging if the coupling quality is below a predetermined threshold.

21. A non-linear acoustic non-destructive testing apparatus for detecting sub-surface discontinuities comprising: at least one transducer; a driving circuit communicably coupled to at least one transducer in use for receiving resultant acoustic signals from the workpiece; and a processor arranged to process data from the output circuit to identify non-linearities in the frequency domain within the resulting signals which may be indicative of defects in the workpiece, characterised in that the apparatus further comprises: an acoustic impedance detector configured to detect and/or monitor the acoustic impedance at the interface between the transducer and the workpiece.

22. A non-linear acoustic non-destructive testing apparatus substantially as herein

described and/or with reference to the accompanying drawings.

23. A method for non-linear acoustic non-destructive testing substantially as herein described and/or with reference to the accompanying drawings.